Is global warming affecting fish growth in the world’s oceans?

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Global warming is not only changing the temperature and the oxygen content of the oceans, but can also significantly affect aquatic productivity and fisheries because of the reduction in size of marine fish species.

An investigation by a team of fisheries biologists from the Fisheries Centre, University of British Columbia in Vancouver, published in the British journal Nature Climate Change suggests that variations in oceanographic conditions caused by global warming could result in a drastic reduction in the growth potential of marine fishes.

The study, led by William W. H. Cheung, sought to model the impact of global warming using two climate scenarios commonly accepted by specialists for the period 2001-2050, on the integrated biological responses of 610 fish species brought about by changes in distribution, abundance and body size.

A school of Blue and Gold Snappers (Lutjanus viridis), at Frijoles, Coiba National Park, Panama.

LASZLO ILYES

Among the results, the scientists conclude that changes in the ocean’s bio-geo-chemical properties, likely to occur within the next 40 years, could alter the marine environment to the extent that fish and marine invertebrates may no longer be able to obtain the energy required for continued growth.

Reduction in dissolved oxygen levels

One of the main factors is a reduction in dissolved oxygen content of the seawater. The capacity of water to hold oxygen to saturation levels, as required to sustain regular physiological adaptations in fish, is among other factors, highly dependent on water temperature. The dissolved oxygen available for fish respiration and metabolic (oxidative) processes in fish is critical for the transformation of food resources into growth and reproduction.

Observed and projected changes in the concentration of oxygen in the ocean (1970-2050). (Modified from Cheung et al, 2012).

Cheung et al. 2012

Reduction in dissolved oxygen affects many physiological, biochemical and behavioural processes in fish including food conversion efficiency, impairs swimming performance, limits growth, and may also cause anomalous development of eggs and larvae.

“Oxygen is one of the key ingredients for body growth. Ample theoretical and empirical evidence suggests that the capacity for growth is limited by oxygen in aquatic organisms and oxygen-limitation is one of the fundamental mechanisms determining biological responses of fish to environmental changes,” say the researchers in their report.

The scientists acknowledge in their report that their assessment has some limitations and they explain the reason why they applied two models in their research: “This study requires a number of assumptions and simplifications to represent and project long-term changes in the complex biological and earth systems, and is thus subject to several sources of uncertainty. There are uncertainties associated with projections of climate and ocean conditions. We attempted to address this by using outputs from two earth system models and identifying areas of agreement between models.”

Reduction in fish growth

The researchers conclude that under the projected oceanographic conditions, the average weight of fish may decrease 14% to 24% between 2001 and 2050. That would be the equivalent to a reduction in average size of humans from about 77 kilos to between 66 and 58 kilos. According to the researchers the areas most affected would be tropical and intermediate latitudinal areas of the Indian Ocean (24%), followed by the Atlantic (20%) and the Pacific (14%). On the basis of previous studies on interactive and cumulative effects of other human stressors in marine systems, the researches anticipate that overfishing and pollution can further aggravate the problem.

Global warming controversy

As most people know, there are differing opinions about the certainty of the occurrence of climate change and global warming. There are those that consider global warming and climate change as a fabrication not supported by scientific evidence and a lie driven by hysteria.

On the other hand, some prestigious researchers contend that global warming is responsible for observed variations in weather patterns including floods and drought conditions, the frequency and severity of tornadoes and the increase in median ocean temperature.

In an opinion article recently published in the Washington Post, James E. Hansen, director of the NASA Goddard Institute for Space Studies, and one of the world's leading climate scientists, states that climate change is a fact, that is here now, and it’s “worse than we thought”.